why does this not work and how do you do it?

[Peter Alcibiades]

One card, one button, one field (called Field)  Traversal is off on the 
button, on for the field. There is only one script - for the button - which 
goes:

on mouseUp
  ask "think of a number"
  if it is not a number then 
  put empty into field "Field"
  focus on field "Field"
  else
  put it into field "Field"
  focus on field "Field"
  end if
end mouseUp

What I expect to happen is that in either case the user will be able to enter 
from the keyboard.  But what actually happens is that the cursor flashes in 
the field, but the keyboard is dead.  To activate the keyboard you have to 
double click in the field (why double?).

There is going to be a simple and obvious answer, but I cannot find it.  Oh 
dear.

Yours, fully expecting to feel very foolish in a few minutes from now....!

Peter